Implantable medical devices (“IMD”) are used today for various applications to deliver electrical pulses from a pulse generator within the IMD, through an electrical lead connected to the IMD, to a targeted location within a patient's body. For example, IMD's may be used in neurological applications, such as for deep-brain stimulation and spinal cord simulation, in which leads deliver electrical pulses generated in the IMD through the electrical lead to targeted portions of a patient's brain or spinal cord. In still other applications, leads may be used to sense particular conditions within a patient's body, and relay that sensed condition back to a processing unit within the IMD.
The electrodes extending from such IMD's are typically introduced through a needle or surgical opening (e.g., a laminectomy or laminotomy) into the epidural space. As they are advanced, it is generally preferable that they proceed in direct contact with the dura, so as to be close to the spinal cord, rather than through the fatty tissue behind the dura, which not only add electrical resistance but also add mechanical resistance. The forces resisting electrode advancement, as the electrode traverses spaces and tissues with different properties, are highly variable. Moreover, fibrous webs and scar tissue, for example, may block the intended path for the electrode, and they may cause the electrode to deviate to one side or the other, or dorsally and away from the dura, and away from the intended path.
Such difficulty in placement of the electrodes creates challenges for the operator, as appropriate electrode positioning is quite important to ensure proper delivery of the intended treatment. The appropriate position for the electrode may be determined by test stimulation, typically in the aware patient, who reports areas of the body where stimulation elicits paresthesia. Thus, one goal of electrode implantation is to achieve the proper distribution of paresthesia by adjusting the left-right and longitudinal position of the electrode. Test stimulation may be delivered intermittently, while the electrode is stationary, rather than continuously. Representative contact combinations may be tested if a multicontact electrode is used. Unfortunately, this process of determining proper electrode placement may be quite time consuming.
In other cases, electrodes may be placed to achieve appropriate radiographic electrode position. Percutaneous electrodes, for example, are routinely positioned under continuous, real-time fluoroscopy, which allows them to be steered as they are advanced in the epidural space. Likewise, insulated paddle electrodes can be positioned under fluoroscopy. In each of these cases, physically steering the electrode to its intended position may be a challenging and tedious process.
In still other cases, electrodes may be placed under direct vision, such as where an electrode is placed via laminectomy. In this case, the dura is directly visualized through a surgical opening, and the electrode is placed through the opening. Typically, the size of the electrode exceeds the size of the opening, and it is advanced through the opening, ascending or descending through the epidural space beneath intact lamina(e), concealing it from view. To the extent that the visible portion of the electrode is connected physically to the invisible portion, the position of the latter must typically be inferred.
In each of these cases, the configuration of the electrode, and particularly of paddle electrodes, makes them difficult to manipulate and control as they are directed to their intended implantation site. More particularly, paddle electrodes typically (i) have limited torsional and bending rigidity, (ii) are difficult to grasp and hold with standard surgical instruments; and (iii) by virtue of their shape and size may respond inconsistently to manipulation. As a result of these issues, they become unwieldy as the tip of the electrode is advanced away from the bony opening.
In order to aid in the placement of such devices, electrode manufacturers may supply “blanks” that are the same size and shape as the electrode, and that may be introduced before the electrode, in order to confirm that the exposure is adequate and that the path for the electrode is clear. Some such blanks may be soft and supple, while others may be relatively rigid (more so than the electrode), and may be used as dissecting tools to open and define the space as necessary. Such blanks and dissecting tools that may be supplied with SCS electrodes unfortunately carry similar disadvantages to those discussed above with regard to the electrodes themselves, and may themselves experience difficulty when meeting an obstruction in the epidural space. There is often a septum of fibrous tissue in the midline of the normal epidural space, and patients who have had previous surgery, epidural injections, and epidural catheter or electrode placement may have epidural scarring. Standard instruments, like standard electrodes, having a rounded tip may, when they encounter an obstruction, deviate to one side or the other, again making control difficult. Even under fluoroscopy, and even with a relatively rigid instrument or electrode, this can be intractable and frustrating for the operator.
It would therefore be desirable to provide an improved electrode, as well as a dissecting and/or electrode placement tool, wherein the design or configuration of the tool itself provides a mechanism that facilitates the process of implantation.